Using optical full-field measurement based on digital image correlation to measure strain on a tree subjected to mechanical load

Determination of Strain Patterns Across the Root-Stem Transition Zone in Trees

“…Also to add the complexity of bark geometry, one must consider the hydro-elastic swelling or shrinkage and the day-night expansion or shrinkage of bark (Bonnesoeur et al 2016 ), of which little information is readily available. Questions currently arise about the use of bark for direct strain measurement found on the xylem of trees, however for this research, we are primarily concerned with mapping strain patterns along a relatively concentrated portion of the tree (the RSTZ), and not determining direct strain values for xylem, while Sebera et al (2014) showed that strain was different as measured on the bark and xylem.…”

“…Static pull tests have been implemented into tree biomechanics and open-grown trees to measure the amount of force required to pull trees to failure (Peltola 2006, James et al 2013a, Smiley 2014, and to test the effects of root loss on short term tree stability (Smiley 2014). Static pulling trials are implemented to test trees nondestructively, (Brudi and van Wassener 2002, James 2013a, Sebera et al 2014 to estimate overall tree stability during formal tree risk assessments (Figure 15). Although it is documented that static loading on tress does not fully consider all the dynamic parameters of wind loading (Moore 2004, Peltola 2006 Tosi (2007) suggested that static tree pulling has been known to overestimate the force required to uproot trees, while Kane (2006) comments that most tree pulling trials have been performed on trees exhibiting little or no visible structural defects, while the amenity trees of most concern are rarely found in pristine condition.…”

“…Strain can be determined utilizing digital image correlation (DIC) techniques, which is widely are used to nondestructively test two and three dimensional surface deformation (Godara et al 2009, Guan et al 2014, Johansson et al 2013, Tyson et al 2002, Sebera et al 2014) on material such as wood (Bjurhager et al 2008). Dahle (2015 Under Review) has found the application of DIC strain mapping can be successfully used to nondestructively observe strain movement in trees, using bark as a surrogate for strain in the xylem.…”

“…Utilizing DIC strain mapping techniques on biological plant organisms, such as trees, repeated testing can occur over a period of time, leaving the tree and associated components in their natural environment ( Figure 19). One advantage of utilizing a nondestructive method for sampling wood and surface strain data from full field measurements is that Finite Element Models data can be directly compared (Tyson et al 2002) and empirical data established by the research community effectively compared (Sebera et al 2014). It is noted in the literature that because of differences in the properties between the white, contrasting base paint layer and the upper surface of the bark, these differences could affect the accuracy of strain mapping and resulting analysis (Cintron and Saouma 2008).…”

“…Tangential root divergence was calculated by the angle of departure from the plane found 90 degrees from the pulling direction ( Figure 32). Once the load cell and all rigging devices and pulling lines were properly installed on the tree for loading, the ARAMIS DIC camera system (GOM, Brauschweig, Germany) was positioned over the respective RSTZ and root, similar to the protocol described in Sebera et al (2014). Refer to Figure 29 for typical static loading setup used for this research.…”